1. Field of the Invention
The present invention relates to a process for the directed vapor deposition of an electron beam evaporant and an apparatus for performing the process.
2. Discussion of the Background
The recent emergence of vapor phase processing technology has led to an atom by atom capability for producing coatings and engineered lamellar materials (already widely practiced in molecular beam epitaxy (MBE) for semiconductor heterostructures and for the production of Al/Al.sub.2 O.sub.3 laminates). Today, the most widely used vapor phase processing technologies are physical vapor deposition (e.g., sputtering, traditional electron-beam evaporation, ion deposition, effusion cells) and chemical vapor deposition (e.g., metal organic, plasma assisted). As noted in the table below, both of these approaches, while useful, have significant drawbacks which prevent or inhibit their use for processing many industrially important materials into desired thin and thick films.
______________________________________ Advantages Problems ______________________________________ Chemical Vapor Deposition (CVD) High throwing power (i.e., not Expensive precursor gases line of site) Many metals, semiconductors High temperature needs and ceramics Uniform deposits Low deposition rates Controllable microstructure Inefficient materials utilization Not all material systems Complex equipment for multi- layers Environmental impact Residual (deposition) stress Physical Vapor Deposition (PVD) All metals and ceramics Expensive equipment Controllable microstructure Low deposition rates (.about.10 .mu.m/h) Inefficient materials utilization (1-25%) High vacuum (10.sup.-7 Torr) needed Line of site deposition Residual (deposition) stress Porosity Batch process ______________________________________
Conventional CVD and PVD have numerous limitations. CVD often requires high temperatures to facilitate gas reactions while being able to deposit materials only slowly and inefficiently. Similarly, PVD processing requires expensive equipment and high vacuum facilities while only being capable of relatively low deposition rates, inefficient materials utilization, and line of site deposition. Traditional electron beam evaporation technology (a type of PVD) has been utilized for materials processing only in high vacuums, precluding its use with a stream of gas to focus and direct the evaporant for efficient materials deposition at extremely high rates. (Within the context of the present application the term "high vacuum" is equivalent to extremely low pressure, such as 10.sup.-6 Torr, while "low vacuum" is equivalent to higher pressures from 0.001 Torr up to but not including atmospheric pressure).
JVD (jet vapor deposition, U.S. Pat. No. 4,788,082), a process which utilizes a stream of gas to direct evaporant material to a substrate, has also fallen victim to similar limitations as CVD and JVD. The JVD process has always created its vapor inside of a jet forming conduit and then directed the gas and evaporant out through a nozzle and onto a substrate. Evaporating inside the gas flow tube is likely to lead to clogging of the nozzle even after short operation times. The inventor of JVD only envisioned making use of fairly rudimentary evaporation techniques such as resistive heating which are incapable of evaporating important refractory elements and compounds, can operate only at low rates (relative to e-beam evaporation), and are likely to contaminate the deposited material as the tungsten filament or its protective sheathing evaporates with the evaporant source material.
The inventor of JVD envisioned using only fairly small nozzles of an exit diameter "from several mm to 2 cm" ("Handbook of Deposition Technologies for Films and Coatings", 2nd ed., Ed. R. F. Bunshah, Noyes Publications, p. 823, 1994). These relatively small nozzles limit the volume of evaporant which may be passed through the nozzle per unit time without the formation of clusters via 3 body collisions. Clusters are groups of atoms which have joined together while in the carrier gas flow. In the Handbook of Deposition Technologies for Films and Coatings the inventor of JVD shows the dependence of clustering upon metal vapor concentration: EQU .tau..sub.38 .ident.10.sup.32 /(M)(He) (sec)
where M and He are the relative concentrations of metal vapor and carrier gas. As the concentration of metal vapor increases the time necessary for cluster formation decreases. While this simple formula provides a general relation between cluster formation rate and metal vapor concentration it neglects the temperature dependence of the phenomenon as noted by Mikami. ("Transport Phenomena in Free-Jet Expansions", H. Mikami, in Bulletin of the Research Laboratory for Nuclear Reactors, vol. 7, p. 169, 1982). Research (D. Hill, Masters Thesis, University of Virginia, p. 59, 1994) has shown this critical temperature dependence makes cluster formation extremely likely using current JVD technology at or near room temperature. Research results presented by Hill indicate that under low Mach flow regimes, as many as 15-20% of all metal atoms can be involved in clustering during deposition at or near room temperature. When these atom clusters are deposited on the substrate they inhibit atomic motion vital to the formation of fully-dense and crystalline deposits which are almost always the preferred end product. To avoid clustering in these conventional JVD systems, evaporation rates must be lowered.
For over thirty years electron beam guns (e-guns) have been recognized and used as superior evaporation tools for producing material vapor streams of new and unusual substances (especially of refractory metals) for deposition upon various substrates. Advantages of electron beam (e-beam) evaporation include high evaporation rates, freedom from contamination, precise beam (power and position) control, excellent economy, and high thermal efficiency. The high evaporation rate and thermal efficiency of e-beam systems are related to the ability of e-guns to bring the heating source (electrons) directly into contact with the vapor emitting surface where beam controls allow precise evaporation rate control. With an e-beam source, the directly heated vapor-emitting surface has the highest temperature of the evaporating assembly, allowing the evaporation of materials from water-cooled crucibles, a near necessity for evaporating reactive, and highly reactive refractory materials. Importantly, the use of the e-beam source with a water-cooled crucible allows "skull" melting/evaporation which prevents crucible wall materials and related reaction products from entering the vapor stream.
Crucibleless (levitation) methods are also available for contaminant-free evaporation. Currently, such levitation methods are used to melt (alloy) small quantities of reactive metal alloys in laboratory environments. They are also used in the aluminum industry to continuously cast metals. In all other heating modes the energy flow goes through the crucible (or resistance-heated wire), then the molten evaporant, and finally to the vapor emitting surface, allowing for significant thermal losses and contamination.
E-beam sources can be used to evaporate many different forms of material, whilst feeding, filling, and changing from one evaporant to another can be easy and continuous. Pure elements, compounds, alloys, and mutually insoluble materials--virtually the entire periodic table of elements in all possible combinations--can be processed by e-beam evaporation. Low vapor pressure elements, such as molybdenum, tungsten, and carbon, are readily evaporated, as are the most reactive elements--titanium, niobium, and tantalum. Even alloys containing materials with significantly varying vapor pressures can sometimes be evaporated successfully. For example, it has been shown by others that seemingly difficult to process Ti alloys (such as those containing vanadium) having elemental vapor pressure ratios as high as 1000:1 can be deposited with the starting alloy's elemental ratios. However, elemental segregation continues to be a problem when working with certain material systems, e.g. Nb-Ti.
Traditionally, e-beam evaporation has been conducted in high vacuum systems (&lt;10.sup.-6 Torr), allowing free propagation of both the electron beam and the vapor stream. The resulting "unfocused" evaporation has always resulted in the waste of significant amounts of "expensive to produce" source material. While the desired substrate may be an array of 150 .mu.m diameter fibers, the vapor from a traditional e-beam source leaves the feed stock with a density distribution often described by a cos.sup.n .theta. function (where n=1, 2, 3, or more) which results in coating nonuniformity for large area arrays and poor materials utilization with fibers arrayed only in the region of roughly uniform flux.
Directed vapor deposition has been employed by others, but only with the use of extremely high temperatures and with vapor phase processing methods other than electron beam (see Kalbskopf et al, U.S. Pat. No. 4,351,267; Ahmed, U.S. Pat. No. 4,468,283; and Schmitt, U.S. Pat. No. 4,788,082).
Kalbskopf et al, U.S. Pat. No. 4,351,267, disclose an apparatus for depositing a layer of a solid material on a heated substrate. Their process uses a directed vapor deposition process using multiple vapor curtains which converge on the surface of the substrate. The coating substrate is provided by reactions of the gaseous reactants contained individually in each of the vapor curtains, which when converged, react to form the coating material.
Ahmed, U.S. Pat. No. 4,468,283 discloses a method for directed CVD which requires collecting and recycling of the vapor stream in order to improve efficiency. Further, as was done by Kalbskopf et al above, Ahmed uses reactant gases which when in contact with one another, react to provide the coating material.
Schmitt, U.S. Pat. No. 4,788,082, discloses a method for vapor depositing using jet stream entrainment in which the depositing material vapor is generated by resistive heating, contact with a heated surface, or by use of a laser. The resulting vapor is contained within the gas jet nozzle, entrained in the carrier gas stream, and then passed through the jet nozzle into the deposition chamber. However, as noted previously, such generation of the depositing material vapor inside the gas jet may cause severe problems with nozzle clogging. Additionally, the Schmitt process can only work with a limited range of elements, at low rates, and most often with fine wire sources.
While CVD and PVD are capable of producing many industrially important materials, sizeable obstacles must be overcome if they are to yield materials that are cost effective solutions to the design engineer's material selection problems. In the case of CVD, very significant costs are associated with the precursor gases and their inefficient deposition. In the case of PVD, inefficient materials utilization and the need for high vacuum are significant cost factors.
To expand the viable applicability of such vapor phase processing technologies, processes are needed which are:
1. Capable of high deposition rates (&gt;100 .mu.m/min over 100's of square centimeters, i.e. grams/min.)
2. Controllable (well defined layer compositions, distinct interfaces, and microstructure)
3. Efficient (near 100%) in materials utilization
4. Flexible (allowing film layer thickness modulation, volume fraction variation, and many materials systems to be codeposited, i.e., to allow alloying and functional grading)
5. Able to produce near net shapes (to achieve economies by combining process steps)
6. Not capital equipment intensive
7. Not operator intensive (continuous, automated and reliable)